Heparin
Heparin/heparan sulfate is a member of a class of polysaccharides known as glycosaminoglycans (GAG). These materials are copolymers of alternating hexosamine and aldouronic acid residues which are found in sulfated forms and are synthesized as proteoglycans. In the compositions of interest herein, heparan sulfate and heparin, the hexosamine which predominates is N-acetylated or N-sulfated glucosamine (GlcNAc and GlcNS). The aldouronic acid is mostly L-iduronic in heparin and mostly D-glucuronic acid in heparan sulfate. Heparan sulfate is commonly considered to have a higher proportion of glucuronic acid than heparin.
Problems of heterogeneity in preparations of heparan sulfate or heparin isolated from tissues make sharp distinctions difficult. Conventional heparin (used as an anticoagulant) has a molecular weight of 5-25 kd and is extracted as a mixture of various chain lengths by conventional procedures. These procedures involve autolysis and extraction of suitable tissues, such as beef or porcine lung, intestine, or liver, and removal of nonpolysaccharide components. The molecular weight of the chains in the extract is significantly lower than the 60-100 kd known to exist in the polysaccharide chains of the heparin proteoglycan synthesized in the tissue. The GAG moiety is synthesized bound to a peptide matrix at a serine or threonine residue through a tetrasaccharide linkage region of the sequence D-GlcA-D-Gal-D-Gal-D-Xyl.fwdarw.protein, which is then elongated at the D-GlcA residue with alternate additions of GlcNAc and GlcA. The polymer undergoes epimerization at certain of the GlcA residues to give IdoA, and subsequent sulfation.
Due to their chemical similarity, isolated "heparin" may contain considerable amounts of what might otherwise be classified as heparan sulfate.